The present invention generally relates to fluidic devices. The invention particularly relates to fluidic devices equipped with microchannels and methods for detection of gas bubbles in a conductive fluid flowing through such a microchannel.
Fluidic microchannels are found in many biological systems and provide high rates of heat and mass transfer in organs such as the brain, lungs, liver and kidneys. The efficient heat transfer provided by small-scale channels is also exploited for high heat flux cooling applications.
A challenge encountered during the development of microchannel-based devices, as an example, for microchannel-based hemodialysis, is detection of gas bubbles in a microchannel that may disrupt fluid flow or cause other undesirable flow conditions within the device. Moreover for certain medical applications such as hemodialysis, there is critical need to detect and avoid bubbles in the blood to prevent air embolisms, which is a potentially fatal complication. Measurement of the gas void fraction in small-scale (micro) channels is essential for predicting two-phase flow, heat transfer, and pressure drop.
Gas void fraction detection has been widely investigated at the macro-scale. When optical access is available, direct visualization is usually performed to detect gas voids. A variety of alternative methods such as radiation, ultrasound, and electrical impedance-based void fraction sensing methods have been developed to enable real-time measurement without optical access. Electrical impedance-based methods, which are well suited for gas void fraction detection in blood and other conductive liquids, encompass three primary implementation approaches: intrusive wire mesh sensors that detect phase (liquid or gas) at discrete locations on a grid over the flow cross-section, non-intrusive electrode-pair sensors flush-mounted in a pipe wall, and impedance tomography sensors that use sets of circumferential electrodes. Electrode configurations generally include helical electrodes wound around the channel, opposing crosswise electrodes, and electrodes placed along the channel length. Recent studies have reported the investigation of crosswise electrode void fraction sensors for medical applications (e.g., hemodialysis), one at the macroscale and one at the microscale level (e.g., dimensions of up to 100 micrometers).
FIG. 1 represents what is herein referred to as “crosswise” sensor 10. Such sensors are described in Valiorgue et al., Design of a non-intrusice electrical impedance-based void fraction sensor for microchannel two-phase flows, Meas. Sci. Technol., 25(9):095301 (July 2014), incorporated herein by reference. The crosswise sensor 10 includes electrodes 16 on oppositely-disposed sides of a microchannel 14 formed in a substrate 12. Impedance is measured across the microchannel 14 between the electrodes 16 as a fluid flows in a flow direction 11 through the microchannel 14. While effective for detecting gas bubble in microchannels carrying a conductive fluid, such sensors may be difficult or impractical to implement in systems comprising an array of closely spaced microchannels.
In view of the above, there is a continuing desire for methods and apparatuses suitable for detection of gas bubbles in microchannels carrying a conductive fluid.